Morphological variations in a tooth family through ontogeny in Pleurodeles waltl (Lissamphibia, Caudata)

Most nonmammalian species replace their teeth continuously (so-called polyphyodonty), which allows morphological and structural modifications to occur during ontogeny. We have chosen Pleurodeles waltl, a salamander easy to rear in the laboratory, as a model species to establish the morphological foundations necessary for future molecular approaches aiming to understand not only molecular processes involved in tooth development and replacement, but also their changes, notably during metamorphosis, that might usefully inform studies of modifications of tooth morphology during evolution. In order to determine when (in which developmental stage) and how (progressively or suddenly) tooth modifications take place during ontogeny, we concentrated our observations on a single tooth family, located at position I, closest to the symphysis on the left lower jaw. We monitored the development and replacement of the six first teeth in a large growth series ranging from 10-day-old embryos (tooth I1) to adult specimens (tooth I6), using light (LM), scanning (SEM), and transmission electron (TEM) microscopy. A timetable of the developmental and functional period is provided for the six teeth, and tooth development is compared in larvae and young adults. In P. waltl the first functional tooth is not replaced when the second generation tooth forms, in contrast to what occurs for the later generation teeth, leading to the presence of two functional teeth in a single position during the first 2 months of life. Larval tooth I1 shows dramatically different features when compared to adult tooth I6: a dividing zone has appeared between the dentin cone and the pedicel; the pulp cavity has enlarged, allowing accommodation of large blood vessels; the odontoblasts are well organized along the dentin surface; tubules have appeared in the dentin; and teeth have become bicuspidate. Most of these modifications take place progressively from one tooth generation to the next, but the change from monocuspid to bicuspid tooth occurs during the tooth I3 to tooth I4 transition at metamorphosis.